A computer system may typically be formed including a plurality of memories arranged in various hierarchical structures. The highest level memories feature the highest performance, the highest price and the lowest capacity, while the lowest level memories have the reverse characteristics to the highest level memories. The highest level memories correspond to a register, a cache, an SRAM, etc., and the lowest level memories correspond to a hard disk drive (HDD), an optical disk drive (ODD), etc.
As a central processing unit (CPU) is generally configured in pipeline architecture, managing such hierarchical memories with efficiency is thus regarded as important in determining the performance of a computer system.